Multi-finger interaction, enabled by many modern computing devices, has changed the manner in which users interact with such computing devices. With multi-finger interaction, a computing device including or operatively coupled to a presence-sensitive device can detect multiple concurrent inputs (e.g., multiple finger inputs) and, in response, modify a graphical user interface output by the computing device for display at a display device. Such support for multiple, concurrent inputs enables a computing device to provide an intuitive, expressive, and dynamic user interface.
However, the richness and intuitiveness of this multi-finger interaction often comes at the cost of increased complexity for developers, who must specify how applications should respond to the multiple concurrent inputs described above. Because event models based on a user interface capable of transitions between multiple fingers and their simultaneous movements are complex, it can be difficult for developers to program, test, and deploy applications supporting this powerful functionality.